Gas detector system



Dec. 2, 1969 R. D. 066 3,482,233

GAS DETECTOR SYSTEM Filed April 13, 1967 2 Sheets-Sheet 1 g lol g IOB RIB INVENTOR.

ROBERT D. 066

BY ATTORNEYS Dec. 2, 1969 R. D. 066 3,482,233

GAS DETECTOR SYSTEM Filed April 15, 1967 2 Sheets-Sheet 2 INVENTOR.

ROB ERT D. 066

BY ATTORNEYS United States Patent O 3,482,233 GAS DETECTOR SYSTEM Robert D. Ogg, Portland, Maine, assignor to The Eastern Company, a corporation of Connecticut Filed Apr. 13, 1967, Ser. No. 630,614 Int. Cl. G08b 21/00; G01n 31/00 US. Cl. 340-237 4 Claims ABSTRACT OF THE DISCLOSURE A gas detector system of the type wherein an atmosphere containing a suspected explosive gas is drawn over a catalytic filament and the resistance of the wire is compared with that of a similar filament not exposed to the suspected explosive atmosphere wherein a differential amplifier is employed to detect very small resistant changes, the differential amplifier feeding into a monostable flipfiop circuit which controls a cut off switch so that even if there is only a momentary rise in resistance caused by an explosive atmosphere, the cut off switch will stay in an off position until manually reset. A time delay device is also incorporated in the circuit so that an adequate test period is provided in the on condition, preventing premature activation of the system being controlled. Also a burn-out warning device is provided in case one or both of the catalytic filaments burns out.

BACKGROUND OF THE INVENTION Field of the invention Explosive atmosphere detecting devices of the catalytic filament type.

Description of the prior art The closest known prior art is the Moore Patent 3,041,- 591 showing a gas detector system of the general type claimed herein. In the Moore system, there is a danger that a false safe reading may be obtained since, if gas containing atmosphere is far above the explosive limits, the catalytic filament, instead of being heated by oxidation of the gas, will actually be cooled since there is insufficient oxygen for combustion, thus giving a false safe reading. Further, in the Moore system, there is no time delay device so that the user may not allow sufficient time for a test. In the Moore system even if the system is properly purged prior to taking a reading, the detector cell may pass through the explosive range so quickly that only a momentary flicker on the indicator occurs, so that the user may miss the warning. After the explosive limit is passed, a false safe reading might be obtained.

SUMMARY OF THE INVENTION A gas detector system wherein a sensitive differential amplifier is used to trigger a monostable flip-flop circuit to actuate a switch cutting off ignition or some similar device until manually reset. In addition, the invention provides a time delay device so that the user is forced to go through an adequate test period prior to activation of the controlled circuit.

BRIEF DESCRIPTION OF THE DRAWING FIGURES 1A and 1B are schematic diagrams illustrating a preferred embodiment of the invention.

In the following description, it will be assumed that the detector is used for the purpose of detecting dangerous fumes in the bilge of a boat. Further, it will be assumed that a cut off circuit is employed cutting off the ignition and starter until safe conditions prevail. However, it should be understood that the invention is one of broad ice applicability and could be used in any environment wherein it is desired to detect an explosive atmosphere and to turn on or off an external circuit in the event that unsafe conditions prevail. Further, it has been assumed that solid state circuitry would be used throughout but it is obvious to those skilled in the art that other electronic devices, such as tubes, might be employed.

In the drawing, the detector cell generally designated 3 is substantially that shown in the Moore patent and comprises a first catalytic filament 5 and a second catalytic filament 7, the first of which is burned in an inert atmosphere and the second of which is burned in the suspected explosive atmosphere. Cell 3 can be located a considerable distance from the area to be monitored and provided with suitable tubing extending into the suspected area. A motor 9 is employed to draw fumes across the element 7 as has been described in the Moore patent. A motor having a permanent field magnet is used so that merely reversing polarity reverses direction. A source of voltage is supplied to the circuit from the conductors 11 and 13, the source of voltage ordinarily being the usual 12 volt battery system. A switch 15 is employed of the four pole, three position type. When the contacts are in the position shown in the drawing, the circuit is off, when the contacts are in the second position, the motor is activated to draw fresh air into the test cell for check or purging purposes as is the electronic circuitry hereinafter described. In the third position, the motor is reversed to draw fumes from the bilge and the electronic circuitry is still on. The function of the fourth pole is to initiate a time delay circuit as later described.

Since many present day systems employ alternators having high charging voltages as well as other devices which might cause voltage spikes, an important aspect of the present invention is the employment of a highly efiicient voltage regulator circuit. This is provided by the Zener diode 17, the usual dropping resistor 19, transistor 21 and the transistor biasing resistor 23. Thus the voltage delivered to point 25 is highly stabilized. In one practical circuit, this value was set at 9.5 volts so that when the system is operating from a 12 volt system, large excursions either up or down can be made in the supply voltage without altering in any substantial manner the delivered to the electronic circuit. The circuit may include a pilot light 27 with its dropping resistor 29 and a second voltage dropping resistor 31 the combination dropping the voltage to the correct value for normal use by the detector cell 3. In addition, a momentary contact switch 41 and an associated dropping resistor 43 can be incorporated in the circuit to bypass the resistor 31 to provide a momentary high voltage across the detector cell 3 so that carbon or other impurities which might collect on the platinum filaments can be burnt off by momentarily applying a higher than usual voltage to the cell.

Output from the cell 3 is applied through line 45 to the base of transistor 47. Similarly a standard reference voltage is applied through line 49 to the base of transistor 51. Variable resistor 50 is used to zero the system in the check position. Because of the efficient VR circuit employed, this can be a factory adjustment. Transistors 47 and 51 constitute a high gain differential amplifier with the differential voltage being applied to and amplified by the transistor 53. Since transistors 47 and 51 constitute the two legs of the differential amplifier, they should be thermally connected as in a dual transistor unit or, less preferably attached to a common heat sink. A meter 55 may be employed to check upon the system although it is not essential to the operation of the system as is hereinafter apparent.

The signal from the collector of transistor 54 is taken through line 57, and passed through a filter composed of resistor 58 and capacitor 60 to prevent erroneous operation by voltage spikes on the supply system and applied to the base of transistor 59. Transistors 59 and 61 constitute a one shot multivibrator or monostable flip-flop wherein transistor 59 is ordinarily in a nonconducting state and transistor 61 in a conducting state. As the input voltage to the base of transistor 59 is raised, transistor 59 will come on, turning ofl? transistor 61 and it will stay off until the momentary contact switch 63 is manually actuated. The output from the flip-flop circuit is passed to the base of transistor 65 which is normally off but, when transistor 61 is turned ott, transistor 65 is turned on, and the output can be used to actuate other devices such as the alarm light 67 and/ or the buzzer 69. Also, the output is fed to the base of transistor 71 and when transistor 65 is turned on, 71 is turned 01?.

The output from transistor 71 is used (together with the time delay device later described) to actuate an interlock relay 75 which can be used to inactivate or activate external circuits. In the embodiment shown, relay 75 is normally open and controls an ignition circuit 77 and a starter circuit 79 preventing starting of the engine until the relay closes. A time delay circuit is provided which is charged by the bottom contacts of switch 15 in the second position. This circuit includes capacitor 81 and resistor 83. Capacitor 81 and resistor 83 have a suitable time constant so that there is adequate time to test and 65 seconds is a suitable value for this delay period. Capacitor 81 remains charged during all the time that switch 15 is in the second position and only begins to decay when switch 15 is moved to the third position. The output from the time delay circuit is passed to the base of transistor 89. Transistors 89 and 91 constitute a monostable flip-flop to give a sharply defined pulse at the end of the delay period. Diode 92 and resistor 93 prevent any false triggering when switch 15is in the second position. The pulse thus generated is passed to transistor 73 turning it on. In order to actuate the relay 75, positive biases must be provided to both transistor 71 and transistor 73.

As an additional precaution, transistor 95 is connected to the time delay circuit to actuate an alarm light 97. Alarm light 97 will stay lit at all times during the time delay period to indicate to the user that the engine can not yet be started, i.e., if either 71 or 73 is oft, 95 will be on.

In order to prevent false activation of the r lay in case one or both of the filaments and 7 should burn out, line 99 leads to a voltage divider comprising resistors 101 and 103 with voltage being taken from the center and applied to the base of transistor 65 through diode 105. If a filament burns out, voltage in line 99 rises, turning transistor 65 on thus activating buzzer 69 and light 67 as well as keeping transistor 71 01f.

The relay 75 is selected so that it will not open after once being operated even should the supply voltage drop to as low as one third of the normal voltage since this is not an unusual condition when the starting motor is actuated with low batteries.

A bypass switch 87 may be provided to override the detector circuit when this is desired. This is a double pole switch, one set of contacts closing relay 75 and the other set actuating buzzer 69 and alarm light 67, preventing accidental operation of the bypass circuit.

The overall operation of the device is as follows. The user first turns switch to the second position which can 4 be described as a check and purge position. In this position motor 9 is operating in the reverse direction, i.e., is pulling fresh air through cell 3 and discharging it into the bilge. In this position capacitor 81 is fully charged, keeping transistor 73 011, preventing actuation of relay 75. One can now feed fumes (e.g., by holding an unlit cigarette lighter near it) into cell 3. If the system is operating properly, meter will be deflected and buzzer 69 and light 67 will go on. As soon as the source of fumes is removed, the system can be restored to normal by pressing reset switch 63, causing the monostable flip-flop (59-61) to reverse. Now the user knows the system is operating properly and can go to the third position of switch 15. In this position, if no gas is detected, transistor 71 will be on since transistor is off. However relay will not be activated until transistor 73 is also turned on by the decay of the delay circuit provided by capacitor 81. If gas is detected at any time during the delay period flip-flop 59-61 will reverse, turning transistor 65 on, causing transistor 71 to be turned off, and activating buzzer 69 and light 67. The circuit will remain in this condition until the trouble is corrected and switch 63 is manually reset.

I claim:

1. In a gas detection system of the catalytic filament, resistance change actuated type having a first reference filament and a second temperature responsive filament, the improvements comprising:

(a) a dilferential amplifier circuit for detecting resistance diiterences in the first and second filaments,

(b) a first monostable flip-flop circuit actuated by said differential amplifier,

(c) an output circuit from said first flip-flop circuit,

(d) a switch actuated time delay circuit,

(e) a second monostable flip-flop circuit, said second flip-flop circuit being actuated by said time delay and having an output circuit,

(f) the outputs of said first and said second flip-flop circuits being efiectively in series and connected to an external circuit whereby,

(g) both of said flip-flops must be actuated to control an external circuit.

2. The structure of claim 1 wherein a reversible motor is employed to actuate a gas pump operatively conn cted to said temperature responsive filament whereby said motor can be actuated in a first direction to draw air over the filament and actuate it in a second direction to draw a suspected atmosphere over said temperature responsive filament.

3. The structure of claim 1 wherein said circuits are activated by solid state devices.

4. The structure of claim 1 wherein a Zener voltage regulator circuit supplies a constant voltage to the system.

References Cited UNITED STATES PATENTS 3,315,245 4/1967 Johnson 340237 3,386,498 6/ 1968 Funfstuck .307273 XR 3,408,853 11/1968 Hiibner 7323 JOHN W. CALDWELL, Primary Examiner DANIEL MYER, Assistant Examiner U.S. Cl. X.R. 7327 

